There are more than half a million pieces of human-made material in orbit around our planet. These materials, referred to as space or orbital debris, range in size from that of a school bus to a thumbtack. Generally space debris is made up of a mix of defunct spacecraft, components of booster rockets and the remains of upper stages of launch vehicles, plus equipment that is lost during spacewalks.
To keep astronauts and spacecraft safe, scientists use radar and telescopes to track debris larger than 10cm and keep a record of these objects and their orbits or trajectories. More than 21,000 objects have been catalogued. This information is used to estimate the number of smaller pieces of debris that cannot be directly detected or monitored, and scientists have estimated the population of particles between 1 and 10 centimetres in diameter to be 500,000, while the number of particles smaller than 1cm could exceed 100 million.
The higher the altitude, the longer the orbital debris will typically remain in Earth’s orbit. Debris left in orbit below 600 kilometres normally falls back to Earth within several years. At altitudes of 800km, the time for orbital decay is often measured in decades. Above 1,000km, orbital debris will normally continue circling the Earth for a century or more.
In low Earth orbit (below 2,000km), orbital debris circles the Earth at speeds of 7km per second. However, the average impact speed of orbital debris with another space object may exceed 10km per second. Consequently, collisions with even a small piece of debris may cause catastrophic damage. Fortunately the probability of two large objects accidentally colliding is low, but can happen as was the case when a Russian satellite accidentally collided with a US satellite in 2009.
So where is the threat? While movies such as Gravity are considered an exaggeration of the threat posed by space debris, the risk is real. Launch vehicles have to have their paths carefully calculated to avoid colliding with the thousands of derelict satellites. Satellites have to be equipped with debris shields. The real threat, however, stems from the ever increasing number of objects being launched into space and the probability that this crowding will lead to collisional cascading, more specifically the Kessler syndrome.
The Kessler syndrome was proposed by the Nasa scientist Donald J Kessler in 1978. It involves a scenario in which the density of objects in low Earth orbit is high enough that collisions between objects could cause a cascade, with each collision generating space debris that increases the likelihood of further collisions. It is a bit of a stretch, but theoretically the distribution and density of debris in orbit could reach a point where space activities and the use of satellites in specific orbital ranges would become unfeasible for generations.
That said, a lot is being done to ensure it doesn’t happen. Through prevention, mitigation and remediation, nations and space programmes are working to slow and eventually reverse the trend of ever increasing quantities of space debris.
Prevention entails ensuring that all future launch vehicles and satellites are equipped with devices that allow them to be disposed of at the end of their life cycles. This means safely directing them into Earth’s atmosphere to burn up. While the technology is still being developed, ideas include devices such as tethers, balloons and solar sails.
Mitigation involves the tracking of all space debris to try to limit the number of collisions as well as venting pressure vessels and fuel tanks and discharging batteries to prevent explosions.
Remediation is by far the most complex solution. It generally refers to the removal and elimination of the garbage that is already in space. The technology and methods by which debris can be removed from orbit are in development, but organisations such as the European Space Agency have already launched programmes to tackle the challenge. The ESA’s Clean Space initiative is studying an active debris removal mission called e. Deorbit, which would target a derelict satellite in low orbit, capture it, then safely burn it up in a controlled atmospheric re-entry.
e. Deorbit aside, for the time being we will have to rely on the United Nations’ active set of non-binding guidelines on space debris mitigation. These are based upon principles such as preventing on-orbit break-ups, limiting the objects released during operations, and removing spacecraft and orbital stages that have reached the end of their mission operations from the useful densely populated orbit regions.
The UAE Space Agency is considering legislation on orbital debris mitigation, mimicking the UN guidelines. Outside of the UAE, policies on space debris vary among the many organisations and governments involved in the space industry. Nasa, Russia, China, Japan, France and the ESA have all issued orbital debris mitigation guidelines. However, private industry is often left to regulate itself. Many firms voluntarily adhere to measures designed to limit orbital debris.
In summary, we still have the ability to clean up the debris that is floating around space. Through the implementation of new technology we will be able to ensure that our grandchildren will continue to benefit from the vast array of services and information provided to us by low Earth orbiting satellites.
Dr Mohammed Al Ahbabi is director general of the UAE Space Agency
